CTIS Spindle Seal
Leaking hub seals allow air to escape from your CTIS system, and gear oil to enter your tires.
This cause of CTIS leaking can be hard to find. To check for this seal, carefully remove the vent line from each Geared Hub and check for air escaping from the hub. Check by slipping one end of a short piece of tube over the vent fitting on the hub and placing the other end in a cup of water.
When you remove the CTIS disconnect valve from the center of the Geared Hub, check for gear oil. A small amount of water is normal, and a few drops of gear oil is not unusual, but a large amount of gear oil is not.